In recent years, displays having not only functions for displaying two-dimensional (planar) pictures but also various additional functions are being proposed or put into practical use. Such displays include 3D displays that are capable allowing the user to perceive stereoscopic pictures. Examples of 3D displays include the time-division three-dimensional displays described in Patent Document 1, Patent Document 2, and Patent Document 3.
FIG. 1 is an explanatory view showing the principles of a time-division 3D display. In order to perceive three-dimensional pictures, the user must wear liquid crystal viewing glasses having an optical shutter function.
Time-division three-dimensional displays alternately display at high speed pictures for the left eye and pictures for the right eye that make up three-dimensional pictures. The right-eye optical shutter of the liquid crystal viewing glasses assumes a transmitting state that transmits light and the left-eye optical shutter assumes a blocked state that blocks light in synchronization with the display of pictures for the right eye. The left-eye optical shutter assumes a transmitting state and the right-eye optical shutter assumes a blocked state in synchronization with the display of left-eye pictures. In this way, pictures that differ are alternately irradiated to the user's right eye and left eye and the user perceives a three-dimensional picture.
In addition, content that is not to be divulged to other people such as secret information or private data may be also be included in the pictures displayed by the display. In the current state of ubiquitous computing in society that has come with the development of information equipment, the prevention of display content being viewed by other people even when in the presence of the general public has become an important problem.
Technology for solving this problem has the picture display apparatus described in Patent Document 4. FIG. 2 is a block diagram showing the configuration of this picture display apparatus.
In FIG. 2, picture information storage memory 202 stores in frame units picture signal 201 that is received as input based on frame signal 203. The picture signal that is stored in picture information storage memory 202 is read two times in a frame interval.
The picture signal that is read first is supplied to synthesis circuit 205 as first picture signal 204. The picture signal that is read second is subjected to a brightness and saturation conversion process by brightness/saturation conversion circuit 206 and then supplied to synthesis circuit 205 as second picture signal 207. By the alternating supply of first picture signal 204 and second picture signal 207 to picture display unit 208 by synthesis circuit 205, picture display unit 208 alternately displays pictures according to first picture signal 204 and pictures according to second picture signal 207.
Viewing glasses shutter timing generation circuit 209 generates viewing glasses shutter drive signal 210 for driving the shutter of viewing glasses 211 based on frame signal 203. Viewing glasses shutter drive signal 210 is a signal that places the shutter of viewing glasses 211 in a blocking state in the intervals in which pictures according to second picture signal 207 are displayed. By driving the shutter of viewing glasses 211 by this viewing glasses shutter drive signal 210, a person wearing viewing glasses 211 perceives pictures according to first picture signal 204.
A person not wearing viewing glasses 211 sees a gray picture in which first picture signal 204 and second picture signal 207 are merged due to the visual time quadrature effect (after-image). This gray picture is a picture that differs from the picture according to first picture signal 204. As a result, a person not wearing viewing glasses 211 is unable to perceive the pictures according to first picture signal 204.
In addition, there may be a third picture signal that differs from the first picture signal and second picture signal. The first picture signal, second picture signal, and third picture signal are displayed in order, and the shutter of viewing glasses 211 is set to the blocking state in intervals in which pictures according to each of the second picture signal and the third picture signal are displayed. In this case, a person not wearing viewing glasses 211 is able to perceive pictures according to the third picture signal. In the following description, pictures according to first picture signal 204 are referred to as private pictures, pictures according to second picture signal 207 are referred to as inverted pictures, and pictures according to the third picture signal are referred to as public pictures.
In the example shown in FIG. 2, the frequency at which sets (frames) of private pictures, inverted pictures, and public pictures are displayed must be at least 60 Hz to suppress the perception of flicker to persons wearing viewing glasses 211 and persons not wearing viewing glasses 211. Essentially, the subframe frequency at which the pictures (subframes) of each of private pictures, inverted pictures, and public pictures are displayed must be at least 180 Hz.
When the subframe frequency is less than 180 Hz, flicker is easily noticeable and the picture quality suffers. Further, the reduction of the visual time quadrature effect results in the user's perception of each of private pictures, inverted pictures, and public pictures individually. As a result, the problem arises that the private pictures become visible even to someone not wearing viewing glasses 211 and the confidentiality of the private pictures is decreased.
In the stereoscopic display described using FIG. 1, the frequency at which sets of right-eye pictures and left-eye pictures are displayed must be at least 60 Hz to suppress flicker. In addition, the subframe frequency at which pictures (subframes) of each of right-eye pictures and left-eye pictures are displayed must be 120 Hz.
In order to display subframes at this high-speed frequency, subframes are preferably transmitted at a frame frequency that is the same as the high-speed frequency when transmitting to a display from a transmission source such as a PC. However, because the upper limit of frame frequency is actually 60 Hz in image transmission systems such as the currently widespread DVI, it is not possible to transmit pictures of a higher frequency.
As a result, in order to realize a display that takes advantage of the above-described visual time quadrature effect, a new image transmission mode that can handle high-speed frame frequencies must be devised or existing channels must be set in parallel.
However, the former solution entails the problem of enormous cost for the new development of chips (transceivers and receivers) or cables of transmission sources or displays or the problem of severe limitation of use due to specialization for high-speed transmission. The latter solution entails the problem of complicated arrangement of cables. Accordingly, neither method can be considered a practical solution.
Patent Document 5 describes a technology that enables the realization of a display that takes advantage of the visual time quadrature effect by using an existing image system to transmit pictures having a high-speed subframe frequency. This technology involves the stereoscopic display by a method that differs from the stereoscopic display of the time-division mode such as shown in FIG. 1 and involves the transmission of a plurality of pictures that are planar pictures and depth pictures using an existing image transmission system. FIG. 3 is an explanatory view showing the transmission method that is used in this technology.
As shown in FIG. 3, two-dimensional pictures and depth pictures are multiplexed by multiplexing means 312 into one large picture and this multiplexed picture that has been multiplexed is transmitted using an existing image transmission standard. In this way, despite the transmission of the multiplexed picture at a frame frequency of 60 Hz, each of the pictures that have been multiplexed into the multiplexed picture are transmitted at a frame frequency equal to or greater than 60 Hz, whereby high-frequency pictures can be transmitted using an existing image transmission system.
Apart from this technology, in recent years liquid crystal display devices such as thin liquid crystal displays or liquid crystal televisions have come into practical use in place of displays or television of the conventional CRT method. In liquid crystal display devices, however, the optical transmittance characteristic response of the liquid crystal is slow with respect to changes in voltage, and the problem therefore arises that when displaying moving pictures that have rapid movement, blurred motion occurs in the displayed pictures and the picture quality suffers.
In particular, due to the high frame frequency in displays of the time-division method that takes advantage of the time quadrature effect of vision, the drop in picture quality caused by the slow response of the optical transmittance characteristic of liquid crystal becomes more prominent.
As a technology for ameliorating this drop in picture quality, a technology called OverDrive (OD) has become widely known. OverDrive is used in, for example, the liquid crystal display device described in Patent Document 6.
FIG. 4 is an explanatory view for describing OverDrive.
In OverDrive, a frame (previous frame) that has been received by tuner 402 is temporarily stored in picture memory 411. Comparison circuit 412 then compares this previous frame that has been stored in picture memory 411 with a frame (current frame) that was next received by tuner 402 for every picture element and converts the picture element value of the current frame.
More specifically, comparison circuit 412 increases the picture element value of the current frame when the picture element value of the current frame is greater than the picture element value of the previous frame, and conversely, decreases the picture element value of the current frame when the picture element value of the current frame is smaller than the picture element value of the previous frame.
For example, when the picture element value of the previous frame is 100 and the picture element value of the current frame is 150 and when the picture element value of the current frame is supplied as output without alteration, the picture cannot be displayed at the brightness that accords with the picture element value “150” because the response of the liquid crystal optical transmittance characteristic is slow. As a result, the picture element value of the current frame is converted from 150 to 180. In this way, the response of the optical transmittance characteristic of the liquid crystal becomes faster and the picture can be displayed at a brightness that accords with the picture element value of 150.
In this type of OverDrive, pictures can be displayed at the desired brightness without raising the response speed of the liquid crystal.